Today, golf balls are generally available as solid and wound balls. Solid golf balls, used by the typical amateur golfer, provide maximum durability and distance. These balls have a core formed of a solid sphere of one or more layers. Typically, these balls have hard cores for high initial velocity and hard covers for low spin and durability.
A number of polymers, such as polybutadiene, natural rubber, styrene butadiene, and isoprene, are used in fabricating the solid cores. Today, golf ball solid cores are predominantly made of polybutadiene. Moreover, in order to obtain the desired physical properties for golf balls, manufacturers have added cross-linking agents, such as metallic salts of an unsaturated carboxylic acid. The amount of cross-linking agent added is typically about 20 to 50 parts per hundred parts of polybutadiene. Most commonly, zinc diacrylate or zinc dimethacrylate are used for this purpose. Of these two cross-linkers, zinc diacrylate has been found to produce golf balls with greater initial velocity than zinc dimethacrylate.
Typically, about 5 to 50 pph (parts per hundred) of zinc oxide (ZnO) is also added to the composition. This material serves as both a filler and an activation agent for the zinc diacrylate/peroxide cure system. The zinc diacrylate/peroxide cure system, which is well known to those of ordinary skill in this art, cross-links the polybutadiene during the core molding process. The high specific gravity of zinc oxide (5.57) can serve the dual purposes of adjusting the weight of the golf ball, in addition to acting as an activation agent.
As zinc oxide is known to be an environmentally unfriendly material, it would be advantageous to eliminate or at least substantially reduce the amount of this material from the manufacturing process. However, when the zinc oxide is eliminated from the composition described above, there is a reduction in cure enhancement, which results in less cross-linking and a corresponding reduction in compression and velocity. This result provides a ball with a softer feel, but the resulting ball has substantially less than the maximum velocity allowed by the USGA standard.
Therefore, it would be advantageous to provide a golf ball core composition with an activation agent other than zinc oxide, i.e., wherein all or at least some of the zinc oxide commonly present was eliminated, which would provide a ball with the lower compression, as noted above, but would maintain the velocity and distance of a high compression ball.
Wound balls, which are generally preferred by better players have higher spin characteristics and softer feel. Wound balls include either a solid rubber or a liquid filled center that is covered by many meters of elastic windings. Such cores are thereafter encased in a cover formed of SURLYN.RTM., polyurethane, or balata rubber. The winding and softer covers provide three-piece balls with higher spin rates and more control for better golfers.
Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers generally strive to produce golf balls with initial velocities that approximate the USGA maximum of 77.7 m/s or 255 ft/s as closely as possible.
Golf ball manufacturers are also concerned with varying the level of the compression of the ball, which is a measurement of the deformation of a golf ball or core in inches under a fixed load. Higher velocity on impact, and, hence, greater distance, is generally achieved by increasing the golf ball compression. Higher golf ball compression also generates a harder "feel". Moreover, harder balls must also have a hard cover to keep the driver spin rate down.
As stated above, better players generally prefer the wound golf balls that have soft covers. The soft cover provides increase spin and feel around the greens. However, these players are traditionally giving up distance off the tee because they generate too much spin.
Golf ball manufacturers are continually searching for new ways in which to provide golf balls that deliver the maximum performance for golfers of all skill levels. They seek to discover compositions that provide the distance performance of hard balls and the feel of approach shot spin of softer balls.